Lithium cobalt oxide based compounds with a cubic secondary phase

ABSTRACT

A lithium metal oxide powder for a cathode material in a rechargeable battery, comprising a core material and a surface layer, the core having a layered crystal structure consisting of the elements Li, a metal M and oxygen, wherein the metal M has the formula M=Co 1−a M′ a , with 0≤a≤0.05, wherein M′ is selected from one or more metals of the group consisting of Al, Ga and B; and the surface layer comprising a mixture of the elements of the core material Li, M and oxygen, inorganic N-based oxides and a cubic phase oxide having a crystal structure with a Fd-3mS space group, wherein N is selected from one or more metals of the group consisting of Mg, Ti, Fe, Cu, Ca, Ba, Y, Sn, Sb, Na, Zn, Zr and Si.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No. 14/125,431, filed Apr. 3, 2014, which application is a National Stage application of International Application No. PCT/EP2012/059986, filed May 29, 2012, which claims the benefit of U.S. Provisional Application No. 61/498,038, filed Jun. 17, 2011.

TECHNICAL FIELD AND BACKGROUND

The invention relates to high voltage stable and high density lithium cobalt-based oxide powderous compounds, containing a core and an electron insulating surface. The compounds may comprise known elements such as Mg, Ti and Al for obtaining improved high voltage electrochemical performances and improved energy density. Also a method to manufacture these materials is disclosed. The lithium transition metal oxide powder can be used as a cathode active material in rechargeable lithium batteries.

Due to their high energy density, rechargeable lithium and lithium-ion batteries can be used in a variety of portable electronics applications, such as cellular phones, laptop computers, digital cameras and video cameras. Commercially available lithium-ion batteries typically consist of graphite-based anode and LiCoO₂-based cathode materials. As today's consumer electronics demand rechargeable batteries with higher energy density, there is a surge towards LiCoO₂-based materials with increased specific capacity for more demanding end applications.

Two common ways to improve the energy density are (a) to increase the charge voltage, typically 4.5V or even 4.6V vs. Li metal when fitted in coin cells, and 4.35V and 4.4V vs. graphite when fitted in full cells, which requires more robust cathode materials which can be charged at higher voltage and (b) to increase the packing density, which requires to increase the particle size of the powder particles. Industrial applicability of these two approaches is however limited by side problems. On the one hand, increasing the charge voltage leads to unstable behavior of the electrode, resulting in cathode degradation linked with electrolyte decomposition. As lithium is removed from Li_(x)CoO₂ (x<1), oxidation of Co³⁺ to an unstable oxidation state Co⁴⁺ follows. The higher the charge voltage, the higher the amount of Co⁴⁺. Large concentrations of Co⁴⁺ increase unwanted side reactions between electrolyte and charged cathode. These side reactions result in poor safety, poor cycling stability at elevated voltage and poor storage properties of charged cathode at elevated temperature. On the other hand, increasing the particle size to increase the packing density impairs power capabilities of rechargeable batteries. In order to meet the power requirements, the battery as a whole and particularly the active cathode material itself must have a sufficient high rate performance. Increasing the mean particle size increases the solid-state lithium diffusion length which eventually results in lowered rate performance.

Careful studying of published results on cathode materials allows better understanding of the limitations of LiCoO₂ based rechargeable lithium batteries. A fundamental limitation of state of the art LiCoO₂-based materials development lies in the Li-excess and particle size dilemma. In WO2010-139404, the authors illustrate the relationship between packing density, mean particle size and lithium excess used for the preparation of state of the art Mg and Ti doped LiCoO₂. In short, the higher the packing density, the higher the particle size and the higher the Li-excess, expressed as Li:Co>>1.00—typically Li:Co is around 1.05—used for the synthesis. The mechanism is based on a so-called “lithium-flux effect” where the Li-excess acts as a flux enhancing the growth of LiCoO₂ particles which eventually increases the packing density. Typical packing densities of ca. 3.70 g/cm³ are achieved for 18 μm particles. The authors also emphasize that large pressed densities are preferable as are obtained with monolithic, potato-shaped and non agglomerated primary LiCoO₂ particles. Use of larger Li:Co excesses to achieve larger monolithic particles results however in poor electrochemical performances, with lower C-rate and lower discharge capacity, which in return cancels energy density gains achieved by increasing the particle size. Such large Li:Co values also increase pH, free base content and carbon content, which impairs safety, storage and bulging properties of charged cathodes. Levasseur, in Chem. Mater. 2002, 14, 3584-3590 established a clear relationship between the increase of structural defect concentrations, as evidenced by means of ⁷Li MAS NMR, and the increase of Li:Co excess.

As a consequence, current state of the art synthesis does not allow to achieve dense, monolithic LiCoO₂-based particles with reduced Li:Co excess. Partial improvements have been achieved but the above basic problems have not yet been fully resolved. Hence there is clearly a need for high capacity LiCoO₂ based cathodes which can be cycled in a stable manner in real cells at higher voltages.

In the prior art several approaches have been suggested to cope with this problem. To achieve high voltage stability, LiCoO₂ materials are usually coated (for example with Al₂O₃) or otherwise chemically modified (e.g. by providing a fluorinated surface). A problem is that coated dense LiCoO₂ often has a lower reversible capacity, so that a part of the gain of energy density by charging to higher voltage is annulled by lower intrinsic capacity. This effect can be observed for aluminum oxide protective and LiF protective coatings, but similar effects are observed for other coating approaches such as ZrO₂, AlPO₄ . . . .

Studying the literature furthermore tells us that coating might not be necessary at all to achieve high voltage stability. Chen & Dahn (Electrochem. Solid-State Lett., Volume 7, Issue 1, pp. A11-A14 (2004)) for example report that a fresh prepared LiCoO₂ could cycle in a stable manner at 4.5V if tested in coin cells with Li metal anodes. Such an approach might be correct for coin cells but the effect cannot be reproduced in real commercial cells. These results are confirmed by the fact that now, several years after the publication, special treated—and not pure—LiCoO₂ is commercially sold for high voltage applications.

Currently no other strategies are known which lead to high voltage performances. It is an object of the present invention to define a cathode material having a high packing density, high rate performance, improved discharge capacity and showing high stability during extended cycling at high charge voltage for high end secondary battery applications.

SUMMARY

Viewed from a first aspect, the invention can provide a lithium metal oxide powder for a cathode material in a rechargeable battery, consisting of a core material and a surface layer, the core having a layered crystal structure consisting of the elements Li, a metal M and oxygen, wherein the metal M has the formula M=Co_(1−a)M′_(a), with 0≤a≤0.05, wherein M′ is either one or more metals of the group consisting of Al, Ga and B; and the surface layer comprising a mixture of the elements of the core material Li, M and oxygen, inorganic N-based oxides and a cubic phase oxide having a crystal structure with a Fd-3mS space group, wherein N is either one or more metals of the group consisting of Mg, Ti, Fe, Cu, Ca, Ba, Y, Sn, Sb, Na, Zn, Zr and Si. In one embodiment, the Li content of the powder, expressed as Li:M, has a value of 0.995 or below. In another embodiment, the Li content is set at a value of Li:M between 0.963 and 0.993. The value of a may be as follows: 0<a≤0.03. The thickness of the surface layer may be less than 100 nm. In an embodiment, N consists of Mg and Ti, the Mg content being between 0.1 and 1 mol % and the Ti content between 0.1 and 0.5 mol %. The N-based oxides may comprise oxides having incorporated Li atoms. It may also be that the concentration of N in the surface layer decreases with increasing distance from the surface of the lithium metal oxide powder.

In the powder, the cubic phase oxide having a crystal structure with a Fd-3mS space group may have a lattice constant “a_(s)” with 8.055≤a_(s)≤8.150, a_(s) being expressed in Å. In an embodiment, the amount of cubic phase oxide in the powder is between 0.1 and 5 wt %. In another embodiment, the powder has an electrical conductivity less than 10⁻⁵ S/cm, the electrical conductivity being measured under an applied pressure of 63.7 MPa. In still another embodiment, the powder has a ⁷Li MAS NMR spectrum that features a unique resonance around −0.5 ppm.

In a further embodiment, the core material is substantially free from the paramagnetic metals Co²⁺, intermediate spin Co³⁺ and Co⁴⁺. In still another embodiment the powder has a bimodal particle shape distribution where the small particle size fraction has a D50≤5 μm and is between 3 to 20 Vol %, and where the large particle size fraction has a D50≥12 μm. It is clear that further product embodiments according to the invention may be provided by combining individual features that are covered by the different product embodiments described before.

Viewed from a second aspect, the invention can provide a method for manufacturing a lithium metal oxide powder for a cathode material in a rechargeable battery, consisting of a core material and a surface layer, the core having a layered crystal structure consisting of the elements Li, a metal M and oxygen, wherein the metal M has the formula M=Co_(1−a)M′_(a), with 0≤a≤0.05, wherein M′ is either one or more metals of the group consisting of Al, Ga and B; and the surface layer comprising a mixture of the elements of the core material Li, M and oxygen, inorganic N-based oxides and either one or both of a cubic phase oxide having a crystal structure with a Fm-3m space group, and a cubic phase oxide having a crystal structure with a Fd-3mS space, wherein N is either one or more metals of the group consisting of Mg, Ti, Fe, Cu, Ca, Ba, Y, Sn, Sb, Na, Zn, Zr and Si, comprising the steps of:

-   -   providing a first mixture of a first Co- or Co and M′-comprising         and a first Li-comprising precursor powder, the first mixture         having a Li:M molar ratio >1.01,     -   sintering the first mixture in an oxygen comprising atmosphere         at a temperature T₁ of at least 600° C., thereby obtaining a         Li-enriched lithium metal oxide compound,     -   providing a second Co- or Co and M′-comprising precursor powder,         and     -   mixing the Li-enriched lithium metal oxide compound and the         second Co- or Co and M′-comprising precursor powder, thereby         obtaining a second mixture wherein the molar ratio Li:M is         ≤0.995, and     -   sintering the second mixture in an oxygen comprising atmosphere         at a temperature T₂ of at least 600° C.;         wherein either one or more of the first Co- or Co and         M′-comprising, the first Li-comprising, and the second Co- or Co         and M′-comprising precursor powders further comprises at least         one element of the group consisting of Mg, Ti, Fe, Cu, Ca, Ba,         Y, Sn, Sb, Na, Zn, Zr and Si. In an embodiment, the first         mixture may have a Li to metal molar ratio between 1.02 and         1.12. In another embodiment the second mixture has a ratio of Li         to metal Li:M between 0.963 and 0.993.

Viewed from a third aspect, the invention can provide an electrochemical cell comprising a cathode comprising the lithium metal oxide powder described before.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B, 1C, 1D, 1E, 1F & 1G: SEM images at 10000× magnification of both prior art LiCoO₂, and material according to the invention.

FIG. 2: XRD pattern of LCO-1 and Example 1. Diffracted intensity is plotted in logarithmic scale as function of 2θ in degrees.

FIG. 3: XRD pattern Example 2. Diffracted intensity is plotted in logarithmic scale as function of 2θ in degrees.

FIGS. 4A & 4B: full scale XPS spectra (10000 Counts per second vs. Binding energy (eV)) for (4A) LCO-1 and (4B) Ex1.

FIGS. 5A, 5B, 5C & 5D: (5A) and (5C) Co2p XPS peaks of LCO-1 and Ex1, respectively. (5B) and (5D) Co3p, Li1s and Mg2p peaks of LCO-1 and Ex1, respectively.

FIGS. 6A, 6B, 6C & 6D: (6A) and (6C) O1s XPS peaks of LCO-1 and Ex1, respectively. (6B) and (6D) C1s XPS peaks of LCO-1 and Ex1, respectively. The peak at 285.5 eV corresponds to the hydrocarbon surface contamination.

FIGS. 7A, 7B & 7C: (7A) and (7B) Ti2p XPS peaks of LCO-1 and Ex1, respectively. (7C) Mg KLL Auger spectrum of Ex1.

FIG. 8: evolution of the Mg, Ti and Si atomic percentage of Ex1 as function of the etching depth normalized using Ta₂O₅ reference.

FIGS. 9A & 9B: ⁷Li MAS NMR spectrum of LCO-1 at (9A) full scale and (9B) 50 times expanded intensity scale (116 MHz, spinning 30 kHz, synchronized echo). Lines marked with asterisks (*) indicate spinning sidebands and are artifacts from the measurements.

FIGS. 10A & 10B: ⁷Li MAS NMR spectrum of Ex1 at (10A) full scale and (10B) 100 times expanded intensity scale (116 MHz, spinning 30 kHz, synchronized echo). Lines marked with asterisks (*) indicate spinning sidebands and are artifacts from the measurements.

FIG. 11: evolution of the magnetization recovery (arbitrary units) as function of the recovery time (s). The solid line is a single exponential fit of the magnetization recovery as function of time I[t]=I[0](1−2*A*exp(−t/T1)).

FIG. 12: ⁷Li MAS NMR spectrum of Ex2 (116 MHz, spinning 30 kHz, synchronized echo).

FIG. 13: ⁷Li MAS NMR spectrum of LCO-3 (116 MHz, spinning 30 kHz, synchronized echo).

FIG. 14: ⁷Li MAS NMR spectrum of Ex3 (116 MHz, spinning 30 kHz, synchronized echo).

FIG. 15: ⁷Li MAS NMR spectrum of LCO-4 (116 MHz, spinning 30 kHz, synchronized echo).

FIG. 16: ⁷Li MAS NMR spectrum of Ex4 (116 MHz, spinning 30 kHz, synchronized echo)

FIG. 17: XRD patterns of samples Ex5-1, EX5-2, Ex5-3 and Ex5-4;

FIG. 18: XRD patterns of samples Ex6-1, EX6-2 and Ex6-3;

FIG. 19: XRD patterns of samples Ex7a, EX7b, Ex7c and Ex7d.

DETAILED DESCRIPTION

In the drawings and the following detailed description, preferred embodiments are described in detail to enable practice of the invention. Although the invention is described with reference to these specific preferred embodiments, it will be understood that the invention is not limited to these preferred embodiments. But to the contrary, the invention includes numerous alternatives, modifications and equivalents as will become apparent from consideration of the following detailed description and accompanying drawings.

The present invention discloses a cathode material having a high packing density, a high rate performance, improved discharge capacity and showing high stability during extended cycling at high charge voltage. This is achieved by a powderous lithium metal oxide consisting of a core material consisting of the elements Li, the metal M and oxygen; and an electron-insulating surface layer consisting of a mixture of the elements of the core materials or inorganic N-based oxides or combination thereof, wherein N is either one or more metals of the group consisting of Mg, Ti, Fe, Cu, Ca, Ba, Y, Sn, Sb, Na, Zn, Zr and Si. The core material has a hexagonal layered crystal structure described as an ordered rock salt-type crystal structure with space group R-3m. The core may be substantially free from structural defects such as oxygen vacancies and Li substitution for M in the MO₂ layers, and also substantially free from paramagnetic metals such as Co²⁺, intermediate spin Co³⁺ and Co⁴⁺.

The existence of a defect-free core is an inherent feature of the cathode material of the actual invention. The inventors observed that a defect-free core, enabling faster diffusion of Li ions into the particles, is related to the high observed rate performance and improved discharge capacity of the disclosed cathode materials.

The surface layer has an inhomogeneous composition compared to the core with a compositional gradient of the different M, M′, Li and O elements. The surface is enriched with elements N such as Mg, Ti, Fe, Cu, Ca, Ba, Y, Sn, Sb, Na, Zn, Zr and Si and, in one embodiment, is formed by segregation from the core and accumulation of these metal dopants at the surface of the particles. In the core, these dopants may be substantially absent. The authors could not indisputably establish the chemical nature of the oxides formed at the surface and therefore speculate that, for example in the case of Mg, Si and Ti doping, possible forms, but not limited to, are LiMO₂, MgO, CoO, Co_(1-φ)Mg_(φ)O with φ≤1, Co₃O₄, Mg_(δ)Co_(3-δ)O₄ with δ≤1, TiO₂, Li₂TiO₃, SiO₂, Li_(ε)Si_(λ)O_(π) with2≤ε≤8, 1≤λ≤2 and 3≤π≤7 . . . . These assumptions are supported by the XPS experiments where the chemical shifts observed for Co, Mg and Ti are typical of oxygen environment and the low electrical conductivity of the particles as the above mentioned oxides are expected to be strong insulators. Where it is said that the surface layer consists of a mixture of the elements of the core material (Li, M, O) and inorganic N-based oxides, by “N-based” oxides are meant also those oxides incorporating Li atoms.

The surface is densely and continuously connected to the core and cannot be physically separated from the particle. Hence in another embodiment the concentration in N metals—with increasing distance from the surface—decreases, possibly in a gradient-like manner and approaches zero in the inside of the particles. The N-enriched surface of the particles feature two additional and unexpected properties:

-   -   (i) The surface is substantially free from lithium salts such as         LiOH and Li₂CO₃. Such characteristic is particularly desirable         in high density high voltage applications such as high-end         polymer or prismatic cells as bulging and storage properties are         considerably improved, and     -   (ii) Surprisingly, the N-enriched surface particles are also         featuring electron-insulating properties. Authors speculate that         the accumulation of oxidized N-based species are responsible for         the low electronic conductivity and provide a physical         separation from the electrolyte, further preventing undesirable         side-reactions.

The surface layer is typically between 20 nm to 200 nm, and preferably between 20 nm and 100 nm thick, and is primarily affected by two parameters:

-   (i) the N content: the thickness increases when the N-content     increases, and -   (ii) the particle size distribution of the powder materials. The     lower the particle size for a given amount of N, the thinner the     surface layer. A too thick layer is not desirable as it results in     increased polarization and eventually in lower rate performance.     Conversely, a too thin layer is also not advantageous as it will     provide bad shielding against electrolyte and be less efficient in     preventing parasitic reactions.

As initially said, an essential feature of LiCoO₂-based cathode materials is a high packing density, which allows increasing the energy density of commercial secondary batteries. In the present invention, a preferred morphology embodiment to achieve high packing density consists in monolithic, potato-shaped and non-agglomerated particles. A monolithic particle does not exhibit inner porosity, and it does not consist of agglomerates of smaller primary particles. A typical particle size (D50) is a least 5 μm or even at least 10 μm, and preferably above 15 μm. The pressed density is typically in the range of over 3.40 g/cm³ and preferably at least 3.70 g/cm³. In one embodiment, the pressed density is as high as 3.90 g/cm³. In another embodiment the pressed density is at least 3.40 g/cm³ for a powder with an average particle size above 6 μm. In still another embodiment the pressed density is at least 3.75 g/cm³ for a powder with an average particle size above 15 μm.

In a process embodiment, a process for manufacturing the high-density and high stability compounds of the present invention runs as follows:

-   (i) providing a first mixture of a first metal M-comprising and a     first Li-comprising precursor powder, the first mixture having a Li     to metal molar ratio >1.01, -   (ii) sintering this mixture in an oxygen comprising atmosphere at a     temperature T₁ of at least 600° C., thereby obtaining a Li-enriched     lithium metal oxide compound; -   (iii) providing a second M-comprising precursor powder, -   (iv) mixing the Li-enriched lithium metal oxide compound and the     second M-comprising precursor powder into a second mixture, whereby     the molar ratio of Li:M in the mixture is adjusted to ≤0.995, and -   (v) sintering the second mixture in an oxygen comprising atmosphere     at a temperature T₂ of at least 600° C. In one particular     embodiment, the metal M=Co.

Later in the text, steps (i) and (ii) are further referred as “the first firing” and steps (iii), (iv) and (v) as the “second firing”. Different implementations of the actual invention are possible, especially regarding the process conditions, the nature of the different precursors and their sequence of blending.

The first M-comprising precursor and second M-comprising precursor can be a mixture of cobalt containing precursor and M′ containing precursor. Examples of suitable cobalt containing precursors include cobalt oxide, hydroxide, oxyhydroxide, carbonate and oxalate.

The M′ containing precursor can be an oxide, hydroxide or organic complex, preferably having sub-micrometric powderous morphology in order to achieve homogeneous distribution and easy blending process.

In several embodiments, either one or both of the first M-, the second M- and the first Li-comprising precursor powders further comprise at least one dopant (M′ or N) from the group consisting of Al, Mg, Fe, Cu, Ti, Ca, Ba, Y, B, Sn, Sb, Na, Ga, Zn, F, P, S, and Zr. In one of these embodiments, either one or both of the first M- and the first Li-comprising precursor powders further comprise at least one element of the group consisting of Mg, Fe, Cu, Ti, Ca, Ba, Y, Sn, Sb, Na, Zn, Zr, F, P, S and Si. A homogeneous distribution of N dopant elements is of primary importance and can be improved by using this process embodiment. In an alternative process embodiment, the homogeneous dispersion of N dopants is improved when the second M-comprising precursor powder further comprises at least one element of the N dopant element group consisting of Mg, Fe, Cu, Ti, Ca, Ba, Y, Sn, Sb, Na, Zn, Zr and Si. Examples of suitable compounds comprising N elements are oxides (such as MgO, TiO₂, SiO₂, . . . ), fluorites (such as MgF₂ . . . ) with sub-micrometric particle size.

In one particular embodiment, Ti and Mg, preferably in the form of TiO₂ and MgO particles with a D50 of respectively less than 100 nm, and less than 1 μm, are added to either one or both of the first and second mixtures described above. In another embodiment, Al, preferably in the form of Al₂O₃ particles with a D50 of less than 100 nm is added to the second mixture described above. In another particular embodiment the Li-enriched lithium metal oxide compound is LiCoO₂ with dense monolithic particles of at least 5 and preferably at least 10 to 20 micrometers. Many commercial prior art LiCoO₂ materials already have this desired morphology.

In yet another particular embodiment the second M comprising precursor features a particle size distribution with a D50 of less than ⅓, preferably less than ¼ of the D50 of the Li-enriched lithium metal oxide compound. In one embodiment, the size ratio between the second M comprising precursor and the Li-enriched lithium metal oxide compound is ⅙. In the latter case and after the second firing, a bimodal distribution is obtained wherein the LiMO₂-based particles originating from the second M comprising precursor are small enough to (a) support a very high C rate and (b) fit nicely in the voids of the larger lithium metal oxide particles packing, which allows for low porosity electrodes and a high volumetric energy density.

The Li-enriched lithium metal oxide compound, referred as the state of the art compound, obtained after the first sintering step, is further characterized by:

-   -   an NMR signal containing at least 2 contributions,     -   large amounts of lithium salts and carbon at the surface of the         particles,     -   an electric conductivity higher than 10⁻⁴ S/cm²,     -   poor electrochemical performances, namely low C-rate and low         discharge capacity.

On the contrary, by finely controlling the lithium stoichiometry, the cathode materials of the present invention, obtained after the second sintering step, are characterized by:—a unique NMR contribution centered around 0 ppm,

-   -   extremely low amounts of lithium salts and carbon at the surface         of the particles,     -   an electric conductivity lower than 10⁻⁵ S/cm,     -   improved electrochemical performances, namely high C-rate and         high discharge capacity.

The authors have observed that if the targeted Li:M is below 0.995, then the materials retain a very good high voltage stability, and the surface is enriched with a secondary phase, which is either one or both of a cubic phase oxide having a crystal structure with a Fm-3m space group, and a cubic phase oxide having a crystal structure with a Fd-3mS space group. When however too much of this secondary phase is present at the surface, there may be less active materials available and the discharge capacity is lowered, and the cell polarization may increase. Therefore an upper limit of 5 wt % of the secondary phase is preferred.

The core of the particles is substantially free from defects. The lithium metal oxide according to the present invention shows electrochemical properties different from those of lithium metal oxides, which are prepared according to state of the art methods, and which contain the same components as the lithium metal oxide of the present invention. This change in physical properties can be observed by ⁷Li-NMR. In ⁷Li-NMR, when a strong magnetic field is externally applied to a lithium-containing material, Li chemical shift values will be shifted due to various hyperfine interactions between a lithium nucleus having a nuclear magnetic moment and the unpaired electrons of metal components contained in the lithium-containing material. The local structural and electronic characteristics of a specific component in the crystal structure of the lithium-containing material can be assessed by measuring the different contributions to the ⁷Li NMR spectrum caused by such chemical shift values.

In a state of the art Ti and Mg-doped lithium cobalt oxide (in the Examples below the materials LCO-1, LCO-3 and LCO-4), prepared according to a conventional method, a sharp peak at around −0.5 ppm in ⁷Li NMR spectra, and additional peaks having their center of mass at around 185 ppm, 5 ppm, −7 ppm, −16 ppm and −40 ppm, are observed (see FIG. 9). In this case, the sharp Li resonance at around −0.5 ppm indicates a Li crystallographic site coordinated only to diamagnetic Co³⁺ metals (t_(2g) ⁶e_(g) ⁰); the electrons of which are all paired. The extra peaks centered at around 185 ppm, 5 ppm, −7 ppm, −16 ppm and −40 ppm indicate Li crystallographic sites partially or fully coordinated to paramagnetic intermediate-spin Co³⁺ (t_(2g) ⁵e_(g) ¹) in addition to the diamagnetic low-spin Co³⁺ metals (t_(2g) ⁶e_(g) ⁰) as discussed in Levasseur, Chem. Mater. 2002, 14, 3584-3590. The interactions between the unpaired electrons of paramagnetic metals and lithium nuclei result in various chemical shifts and allow to reveal different and multiple Li site environments for LCO-1, LCO-3 and LCO-4. The authors speculate that the presence of dopants N, such as Mg and Ti, in the core of the LiMO₂ particle (as was established in the literature by other techniques such as X-ray diffraction by Wenbin Luo et al., J. Electrochem. Soc. 2010, 157, 782) will change the spin state and valency of cobalt ions (as it is expected for Mg²⁺ or Ti⁴⁺ substitution for low spin Co³⁺) or introduce other electron spin carriers (such as Ti³⁺) further increasing the concentration of structural and paramagnetic defects in the core.

In comparison with this, it can be seen that the lithium metal oxide of the present invention shows a unique Li peak around −0.5 ppm though having the same components and composition as those of a lithium metal oxide prepared according to a state of the art method. Consequently, the Examples according to the invention contain only one Li site environment solely surrounded by diamagnetic trivalent metal ions such as Co³⁺ (t_(2g) ⁶e_(g) ⁰) or Al³⁺ (2p⁶). The core of the Example materials is therefore substantially free from paramagnetic impurities, structural defects and dopants N such as Mg²⁺, Ti³⁺ or Ti⁴⁺.

This observation is further confirmed by the measurement of the T1 spin-lattice relaxation time which clearly evidences that paramagnetic spin concentration is larger in the state of the art LiCoO₂-based materials when compared to the cathode materials of the present invention.

When dopants N are present, the surface is formed during the second sintering by spontaneous segregation of the dopants from the core. The exact mechanism of this “in-situ” coating is unknown but the authors assume that a cooperative reaction occurs when the lithium content of the core is adjusted in the second firing step, and during this firing step N dopants such as Mg and Ti are expelled and accumulated at the surface of the particles. As mentioned above, this is corroborated by NMR observations.

Linked to this, another important feature of the cathode materials of the present invention are their “insulating” nature. These cathode materials have conductivities which are at least 2-3 orders lower than those of the least conductive currently known cathode materials. For example, commercial LiCoO₂ has a relatively high electrical conductivity in the range of 10⁻² to 10⁻³ S/cm. That the insulating cathodes of the present invention can yield excellent electrochemical performance, namely a large discharge capacity and C-rate performance, is a surprise because it is commonly accepted that high electrical conductivity is needed for the Li⁺ cation diffusion within the solid cathode and across the interface between electrolyte and cathode.

It is believed that the low conductivity provided by the surface layer is the main reason for the high voltage stability of the cathode materials of the present invention. When a prior art LiCoO₂-based cathode is charged to high voltage—meaning the cathode is strongly de-intercalated—we achieve a Li_(x)CoO₂ (x<<1) composition where most of the cobalt ions are in the 4+ valence state. Tetravalent-cobalt containing Li_(x)CoO₂ is a very strong oxidizer and is highly reactive. The electrolyte becomes thermodynamically unstable when in contact with such an oxidizing surface. A reaction with the electrolyte (being the reducing agent) is strongly preferred energetically. Even at low temperature—during normal cycling of a LiCoO₂ cathode at high voltage—this reaction proceeds slowly but continuously. Reaction products cover the cathode surface and the electrolyte is decomposed, and both effects continuously cause a deterioration of the electrochemical performance of the battery. Also, a loss of capacity and a strong increase of resistance—by polarization—is observed.

Obviously, a cathode material protected by an insulating surface layer will solve this problem by physically separating tetravalent cobalt ions from the electrolyte and eventually preventing further electrolyte reduction. By careful selection of the compounds, such as Mg, Zr, Si and Ti, the process allows to achieve an in-situ coating of the final powder with a layer that is enriched in oxidized compounds, like MgO and TiO₂, that may also be lithiated. This inert coating layer provides additional safety when the powders are in contact with the electrolyte of the battery.

The following examples illustrate the invention in more detail:

EXAMPLE 1

The characterization of Examples 1 and 2 will demonstrate that LiCoO₂-based cathode materials comprising a core where Li atoms occupy a single site surrounded by trivalent diamagnetic metals, and having an electron insulating surface comprising the elements of the core material (Li, Co) and inorganic metal oxides comprising Mg and Ti show improved features for high voltage applications.

Preparation of LCO-1:0.25 mol % titanium and 0.5 mol % magnesium doped Co(OH)₂ as precursor for LiCoO₂ is prepared in a pilot line according to the process explained in WO2010-139404. A state of the art titanium and magnesium doped LiCoO₂ (noted LCO-1) is obtained by means of a standard high temperature solid state synthesis (=the first firing step) by mixing the precursor with Li₂CO₃. Typical Li:Co molar ratio used in the Li₂CO₃-doped Co(OH)₂ blend is 1.06 to 1.12. The average particle size of LCO-1 is 20 μm. The final Li:Co mol ratio of LCO-1 after firing measured by ICP is 1.053 and will be used to determine Li:Co ratio for examples 1 and 2. LCO-1 is also referred to as the lithium doped cobalt oxide “parent” of Example 1 and Example 2.

Preparation of Example 1 (noted Ex1): 95 mol % of LCO-1 and 5 mol % of 0.25 mol % Ti and 0.5 mol % Mg doped Co(OH)₂; corresponding to respectively 95.24 wt. % and 4.76 wt. %; are mixed in order to target a final Li:Co mol ratio of 1.000. The mass of reagents has been calculated assuming a cobalt weight content in LCO-1 and Co(OH)₂ of respectively 60.21 wt. % and 63.40 wt. %, resulting in an absolute error of less than 0.2% in the calculation of Li:Co. The homogeneous mixture is placed in an alumina crucible and heated (=the second firing step) at 925° C. for 12 hours under constant air flow. After cooling, the resulting powder (Ex1) is sieved and characterized. The average particle size of Ex1 is found to be 20 μm.

EXAMPLE 2

Preparation of Example 2 (noted Ex2): 94 mol % of LCO-1 and 6 mol % of 0.25 mol % Ti and 0.5 mol % Mg doped Co(OH)₂; corresponding to respectively 94.28 wt. % and 5.72 wt. %; are mixed in order to target a final Li:Co mol ratio of 0.990±0.002. The homogeneous mixture is placed in an alumina crucible and heated at 925° C. for 12 hours under constant air flow. After cooling, the resulting powder (Ex2) is sieved and characterized. The average particle size of Ex2 is found to be 20 μm.

EXAMPLE 3

The characterization of Example 3 will demonstrate that Li-stoichiometry controlled LiCoO₂-based cathode materials, comprising a core where Li atoms occupy a single site surrounded by trivalent diamagnetic metals, where Co is partly substituted by Al³⁺, and having an electron insulating surface comprising the elements of the core material (Li, Co and Al) and inorganic metal oxides comprising Mg and Ti, show improved high voltage features and large pressed densities.

Preparation of LCO-3: state of the art titanium and magnesium doped LiCoO₂ (noted LCO-3) is obtained by means of a standard high temperature solid state synthesis by dry-mixing powders of Co₃O₄ with TiO₂, MgO and Li₂CO₃. LCO-3 contains 0.25 mol % titanium and 0.25 mol % magnesium. Typical Li:Co molar ratio used for the blend is 1.10. The average particle size of LCO-3 is 18 μm. The final Li:Co mol ratio of LCO-3 after firing—as measured by ICP—is 1.070, and is used to set the Li:(Co+Al) ratio for Example 3.

Preparation of Example 3 (noted Ex3): A cathode powder material is prepared by mixing 85.40 wt. % of LCO-3, 10.51 wt. % Co₃O₄ with mean particle size of 3 μm, 3.47 wt % Li₂CO₃, 0.05 wt % of MgO, 0.05 wt % of TiO₂ and 0.52 wt % Al₂O₃, so as to adjust the Al:Co mol ratio to 0.01:0.99 (or 1 mol % Al substitution on Co sites) and the final Li:(Co+Al) molar ratio to 1.000±0.002. The homogeneous mixture is placed in an alumina crucible and heated at 980° C. for 12 hours under constant air flow. After cooling, the resulting powder (Ex3) is sieved and characterized. The average particle size of Ex3 is found to be 16 μm, and a bimodal distribution is obtained, as shown in FIG. 13.

EXAMPLE 4

The characterization of Example 4 will demonstrate that LiCoO₂-based cathode materials comprising a core with Li stoichiometry control, where Li occupy a single site surrounded by trivalent diamagnetic metals, and having an electron insulating surface comprising the elements of the core material (Li, Co) and inorganic metal oxides comprising Mg and Ti, are suitable for high power applications where maintaining high C-rate and high average voltage are required.

Preparation of LCO-4: state of the art titanium and magnesium doped LiCoO₂ (noted LCO-4) is obtained by means of a standard high temperature solid state synthesis by dry-mixing powders of Co₃O₄ with TiO₂, MgO and Li₂CO₃. LCO-4 contains 0.18 mol % titanium and 0.40 mol % magnesium. Typical Li:Co molar ratio used for the blend is 1.03. The average particle size of LCO-4 is 6 μm. The final Li:Co mol ratio of LCO-4 after firing—as measured by ICP—is 1.015, and is used to set the Li:Co ratio for Example 4.

Preparation of Example 4 (noted Ex4): 98.5 mol % of LCO-4 and 1.5 mol % of Ti (0.18 mol %) and Mg (0.4 mol %) doped Co₃O₄; corresponding to respectively 98.77 wt. % and 1.23 wt. %; are mixed homogeneously in order to target a final Li:Co mol ratio of 1.000±0.002. The mass of reagents has been calculated assuming a cobalt weight content in LCO-3 and Co₃O₄ of respectively 60.21 wt. % and 73.42 wt. %. The mixture is placed in an alumina crucible and heated at 1000° C. for 12 hours under constant air flow. After cooling, the resulting powder (Ex4) is sieved and characterized.

Characterization of Examples 1-4

Changes in physical and electrochemical properties of the lithium cobalt based oxides prepared according to the present invention have been characterized before and after the second firing used to control and adjust the Li:M stoichiometry.

SEM Analyses

Surface imaging is performed using a scanning electron microscope (SEM) and is shown on FIG. 1. All the samples feature monolithic particle morphology before and after the second firing. Following the second firing step, the surface of the particles undergoes clear changes: the surface of LCO-1, LCO-3 and LCO-4 are covered with dust and debris, which contrasts with the smooth surface of Examples 1 to 4 after the second firing.

XRD Analyses

The crystal structure of the lithium cobalt based oxides has been investigated by means of X-ray diffraction. XRD patterns of LCO-1 and Example 1 are shown on FIG. 2. All peaks are indexed in the R-3m space group using a rhombohedral cell with usual lattice parameters a=2.815 Å and c=14.05 Å typical of layered LiCoO₂ phase. No secondary phases, namely cobalt based oxides Co₃O₄ and CoO, are observed. Likewise, XRD patterns of LCO-3, LCO-4, Ex3 and Ex4 are interpreted using the same structural model and similar lattice parameters. Though containing large surface base contents such as LiOH and Li₂CO₃, LCO-1, LCO-3 and LCO-4 XRD patterns don't allow identifying such compounds, suggesting that their content is below XRD detection limit and/or that the surface base are in amorphous forms.

The XRD pattern of Ex2 is shown on FIG. 3. In addition to the main peaks attributed to the layered LiCoO₂-based oxide (R-3m space group), peaks marked by asterisks indicate the presence of a cobalt-based spinel phase (Co₃O₄ indexed in Fd-3mS space group with a=8.085 Å) amounting to ca. 1.2 wt %, which is in very good agreement with the targeted Li:Co mol ratio of 0.990. Ex2 therefore comprises ca. 99 mol % LiCoO₂-based material and ca. 1 mol % cobalt spinel-based phase.

For Ex3, the homogeneous substitution of Co by Al is confirmed by the increase of the ‘reduced’ c/a ratio defined as (c/(√24a)−1)×1000, being 18.56 and 18.90 for LCO-3 (no Al) and Ex3 (containing 1 mol % Al), respectively. The increase of the c/a ratio upon Al doping of LiCoO₂ is in good agreement with other works (such as Myung et al., Solid State Ionics Volume 139, Issues 1-2, 2 Jan. 2001, Pages 47-56 and Gaudin et al., J. Phys. Chem. B 2001, 105, 8081-8087).

X-ray Photoelectron Spectroscopy (XPS) Analysis

X-ray photoelectron spectroscopy (XPS) measurements are carried out using a Kratos Axis Ultra spectrometer fitted with a focused monochromatized Al Kα radiation (hu=1486.6 eV). For the Ag 3d_(5/2) line, the full width at half-maximum is 0.58 eV under the recording conditions. The analyzed area of the sample is 300×700 μm². Peaks are recorded using a constant pass energy of 20 eV. The pressure in the analysis chamber is ca. 5×10⁻⁷ Pa. To prevent the sample from moisture and air exposure, sampling is performed in an argon dry box which is directly connected, through a transfer chamber, to the XPS spectrometer. Short acquisition time control spectra are recorded at the beginning and at the end of each experiment to confirm the non-degradation of the samples. The binding energy scale is calibrated from LiCoO₂ peaks (Co2p, Co3p and O1s). Core peaks are analyzed using a non-linear Shirley-type background. The peak positions and areas are optimized by a weighted least-square fitting method using 70% Gaussian and 30% Lorentzian line shapes. Quantification was performed on the basis of Scofield's relative sensitivity factors. For depth profiling experiments, the depth has been calculated relatively to a sample of Ta₂O₅ for which an argon etching speed of 0.32 nm/s has been observed.

The chemical composition of the surfaces of the particles of LCO-1 and Ex1 has been investigated by means of XPS. Full scale XPS spectra are shown on FIG. 4(a)&(b) and quantitative results for LCO-1 and Ex1 are listed in Table 1.

TABLE 1 elemental composition of the surface of the grains of LCO-1 and Ex1. at. % LCO-1 Ex1 Li 21.7 15.3 Co 8.7 12.0 O 41.9 37.5 C 23.1 26.5 S 1.3 0.8 Ca 0.6 0.1 F 0.9 0.4 Ti 0.9 1.1 Na 0.3 0.3 Si Below detection limit 1.1 P 0.4 0.6 Mg Below detection limit 4.4

The cobalt 2p and 3p XPS peaks are shown on FIG. 5 (a)-(d). Spin-orbit coupling splits the Co2p spectra into two components—2p_(3/2) and 2P_(1/2)—with an intensity ratio of about 2:1. Each component presents a main line at 780 and 795 eV and a ligand-to-metal charge transfer satellite peak at 790 and 805 eV. The Co3p spectrum also consists of a main line and a satellite at 61 and 71 eV, respectively; 3p_(3/2) and 3P_(1/2) energy splitting is however too small to be observed. The Co2p and Co3p spectra of LCO-1 and Ex1 are characteristic of Co³⁺ ions of LiCoO₂ and exclude the presence of Co²⁺ at the surface of the particles. These observations are in line with previous studies on LiCoO₂. (e.g. Daheron et al., J. Phys. Chem. C 2009, 113, 5843).

The XPS analysis is however revealing clear differences in the chemical composition of the surface of the particles of LCO-1 and Ex1:

-   -   Lithium 1s, oxygen 1s and carbon 1s XPS spectra are shown on         FIG. 5 (b) & (d) (Li1s) and FIG. 6 (a) & (c) (O1s) and FIG. 6         (b) & (d) (C1s). Intense peaks from Li₂CO₃ are observed for         LCO-1 with Li1s, O1s and C1s characteristic binding energies         centered on 55.7 eV, 532 eV and 290.2 eV, respectively. Li₂CO₃         is estimated to account for 26 at % at the surface of LCO-1         particles. These contributions are not present in Ex1,         suggesting that the surface of the particles is mostly free from         lithium salts.     -   A well resolved Mg2p XPS peak around 50 eV is observed only for         Ex1 as shown on FIG. 5(d). Though LCO-1 and Ex1 feature         identical Mg:Co ratio equal to ca. 0.005, the Mg:Co ratio         measured at the surface of Ex1 is about 0.37, showing an         enhancement by about 2-orders of magnitude over LCO-1. The         MgO-like environment is confirmed by the typical Mg KLL Auger         structure (cfr. Davoisne et al, Astronomy and Astrophysics,         2008, 482, 541) observed in the 300-400 eV range, as shown on         FIG. 7(c). The absence of magnesium at the surface of LCO-1         suggests that Mg remains within the structure in LCO-1.     -   FIGS. 7 (a) and (b) shows XPS spectra of Ti2p with two         components Ti2p_(3/2) and Ti2p_(5/2) at 458.5 and 464 eV,         respectively for both samples. These binding energies are in         good agreement with the exclusive presence of Ti⁴⁺ in six-fold         oxygen environments (see also: Tanaka et al., Corrosion Science,         2008, 50, 2111 and El Ouatani et al., Journal of The         Electrochemical Society, 2009, 156, A4687). Possible host         structures for Ti⁴⁺ in the present systems are TiO₂ and Li₂TiO₃.         Likewise, the Ti:Co ratios measured at the surface of LCO-1 and         Ex1 are close to 0.1 which is more than 40 times the Ti:Co blend         ratio (ca. 0.0025). The Ti amount at the surface of Ex1         particles is also slightly higher when compared to LCO-1.

The evolution of Mg and Ti as function of particle depth has been monitored for Ex1 by means of XPS depth profiling as shown on FIG. 8. The concentration of Mg and Ti decreases rapidly over the first 50 nm of the particles. The amounts of Mg and Ti do not decrease down to zero, as could be expected, even not after a long time of etching. This is due to side effects of argon ion sputtering, where the implantation of argon ions deep inside the sample results in the forced mixing of atoms of subsequent layers. The larger content of Mg and Ti of Ex1 compared to LCO-1 and the depth profiling experiments suggest an in-situ coating mechanism where Mg and Ti are expelled from the lithium cobalt oxide bulk structure during the Li:Co equilibration (second) firing and are accumulated at the surface of the LiCoO₂ particle in oxidized form. The in-situ segregation mechanism of Mg and Ti will be further evidenced by means of electrical conductivity measurements.

Electrical Conductivity

Measurement of the electrical conductivity is performed at 25° C. under an applied pressure of 63.7 MPa in the 4-probe configuration. In the description and claims the value of 63 MPa is also mentioned as round-off, when the actual pressure of 63.7 MPa is applied. The electrical conductivity of the different materials is listed in Table 2.

TABLE 2 electrical conductivity Conductivity Sample (S/cm) LCO-1 4.50 × 10⁻³ Ex1 6.37 × 10⁻⁶ Ex2 7.32 × 10⁻⁶ LCO-3 8.70 × 10⁻³ Ex3 1.52 × 10⁻⁶ LCO-4 1.30 × 10⁻³ Ex4 2.45 × 10⁻⁶

The electrical conductivity of Ex1 to 4 is three orders of magnitude lower than for the corresponding lithium doped cobalt oxides parents LCO-1, LCO-3 and LCO-4. It is the author's opinion that the lower conductivity originates from two contributions:

-   -   i) the reduction of bulk structural defects, as evidenced by ⁷Li         NMR (see below), enhancing the band insulator properties of Li         stoichiometric LiCoO₂ with Li:Co=1.00, and,     -   ii) in-situ coating of LiCoO₂ particles with insulating Co, Mg         and Ti oxide-based species occurring during the second firing of         Ex1, Ex2, Ex3 and Ex4. The latter is supported by the very low         electrical conductivity of commercially available MgO (Kyowa         Chemicals) and TiO₂ (Cosmo Chemicals KA300), respectively         measured to be below 10⁻⁸ S/cm and 6.02*10⁻⁷ S/cm. As is         relevant for Ex2 containing a cobalt based spinet secondary         phase, it is commonly accepted that the conductivity of Co₃O₄ is         below 10⁻⁶ S/cm.         ⁷Li MAS NMR

⁷Li magic angle spinning (MAS) NMR spectra is recorded on a Bruker 300 Avance spectrometer at 116 MHz (7.05 T magnet), with a standard 2.5 mm Bruker MAS probe. A combination of single-pulse and Hahn echo sequences is used in MAS conditions (30 kHz spinning speed). The single-pulse sequence with t_(π/2)=2.0 μs requires a first-order phasing process with a sin(x)/x baseline correction due to the dead time of the spectrometer. The rotor-synchronized Hahn echo sequence [t_(π/2)−τ₁−t_(π)−τ₂] (with τ₁=τ₂ being equal to one rotor period, i.e. 33.33 microseconds) is used to facilitate the phasing of all the signals and to ensure the observation of possible very wide signals which are lost during the receiver dead time, while refocusing the interactions with electron spins. The 90° pulse duration is equal to t_(π/2)=2.0 μs. A recycle time of 100 s is used. The isotropic chemical shifts, expressed in ppm, have been obtained using 1M LiCl dissolved in H₂O as external reference.

T1 spin-lattice relaxation times are measured using the inversion-recovery sequence on static samples. The T1 is determined by single exponential fitting of the magnetization recovery intensity as function of the recovery delay (from 100 μs to 100s). ⁷Li magic angle spinning nuclear magnetic resonance (MAS NMR) spectra of the different Examples are displayed on FIG. 9 for LCO-1, FIG. 10 for Ex1, FIG. 12 for Ex2, FIG. 13 for LCO-3, FIG. 14 for Ex3, FIG. 15 for LCO-4 and FIG. 16 for Ex4.

One can distinguish two types of patterns:

-   -   LCO-1, LCO-3 and LCO-4 have a complex ⁷Li MAS NMR pattern with         multiple contributions. In addition to the main diamagnetic         contribution centered on ca. −0.5 ppm and related spinning         sidebands, LCO-1 spectra feature multiple paramagnetic         contributions at ca. 185 ppm, 5 ppm, −7 ppm and −16 ppm denoting         several different paramagnetic electron-spin metal environments         for lithium ions.     -   On the other hand, Ex1, Ex2, Ex3 and Ex4 feature a unique Li         resonance centered on −0.5 ppm±0.25 ppm and related spinning         sidebands.

The sharp peak centered on −0.5 ppm is attributed to lithium ions surrounded by only trivalent diamagnetic Co³⁺ (t_(2g) ⁶e_(g) ⁰) ions as reported in literature (cf. Levasseur et al., Solid State Ionics 2000, 128, 11). A unique Li site solely surrounded by diamagnetic trivalent metal ions is therefore observed for Ex1, Ex2, Ex3 and Ex4.

For LCO-1, LCO-3 and LCO-4, additional paramagnetic contributions at ca. 185 ppm, 5 ppm, −7 ppm, −16 ppm and −40 ppm are resulting from structural defects induced by two main contributions:

-   -   lithium overlithiation of layered lithium cobalt-based oxides         favors, for local charge conservation, the presence of oxygen         deficiency leading to Co³⁺ ions occupying square-based pyramids         and having intermediate paramagnetic spin state configuration,         with unpaired electron in the e_(g) orbital as described in S.         Levasseur, Chem. Mater. 2003, 15, 348-354, and,     -   effect of dopants inducing the presence of cobalt ion with         different spin and valence states. For example, recent works         evidenced that Mg²⁺ substitution for Co³⁺ in LiCoO₂ results in         substantial oxygen deficiency (see: Wenbin Luo et al., J.         Electrochem. Soc. 2010, 157, 782). Likewise, this oxygen         deficiency favors the presence of paramagnetic intermediate spin         state Co³⁺ ions. It can also be reasonably assumed that Ti         substitution for Co will either induce Ti³⁺ paramagnetic         impurities or Co²⁺ paramagnetic impurities when present in Ti⁴⁺         valence state, because of local charge conservation reasons.

These examples demonstrate that the doped lithium cobalt based oxides featuring a ⁷Li MAS NMR spectrum characterized by a single Li contribution centered around −0.5 ppm and an increased T1 spin-lattice relaxation time (see below) is also characterized by a high voltage cycling stability. The improved stability, single contribution ⁷Li MAS NMR spectrum and longer T1 are obtained by optimizing lithium to metal ratio. The ⁷Li MAS NMR spectrum of Ex2, indicating the presence of a spinel-based secondary phase also features a unique resonance around −0.5 ppm. This characteristics clearly establishes that Ex2, having a Li:Co=0.99, comprises a LiCoO₂ core where Li ions accommodate a trivalent diamagnetic metal surrounding and a lithium-free spinet phase, most probably present at the surface of the particles.

In addition, substitution of the Co³⁺ by Al³⁺, both trivalent diamagnetic metal ions, does not modify the ⁷Li MAS NMR signal of Ex3, which conserves a unique resonance around −0.5 ppm, unambiguously confirming that lithium ions occupy a unique site solely surrounded by trivalent diamagnetic metals. Likewise, this finding can be extended to substitution of Co³⁺ ions by trivalent diamagnetic metals such as Al³⁺, Ga³⁺ and B³⁺ that will not modify the ⁷Li MAS NMR signal of Li-stoichiometric LiCo_(1−a)M′_(a)O₂ with M′=Al, B and Ga.

The relative absence of structural defects is further characterized by measuring the T1 spin-lattice relaxation time as shown in Table 5. The T1 value of LCO-1, LCO-2 and LCO-4 is not defined and cannot be accurately determined due to several spin-lattice relaxation mechanisms. The characteristic relaxation time of each individual mechanism is however shorter than 0.1 s. On the other hand, T1 values for Ex1-4 are successfully determined by a single exponential fit of the magnetization recovery as shown on FIG. 11.

Both the single ⁷Li MAS NMR contribution and longer T1 value demonstrate the relatively low concentration of structural defects in Ex1, Ex2, Ex3 and Ex4 when compared to LCO-1, LCO-3 and LCO-4.

TABLE 5 Spin-lattice relaxation time T1 values and ⁷Li MAS NMR contributions for the different samples Number of NMR T1 Name contributions value (s) LCO-1 Multiple Not defined Ex1 Single 1.1 Ex2 Single 1.0 LCO-3 Multiple Not defined Ex3 Single 0.3 LCO-4 Multiple Not defined Ex4 Single 0.4

In conclusion, ⁷Li MAS NMR, XPS analyses and electrical conductivity allow to make an unambiguous description of the structure of Ex1, Ex2, Ex3 and Ex4 materials which comprises a core with Li stoichiometry control where Li occupy a single site surrounded by trivalent diamagnetic metals such as low spin Co³⁺ (t_(2g) ⁶e_(g) ⁰) and Al³⁺, and an electron insulating surface comprising inorganic metal oxides comprising Mg, Ti, Si, Co and Li.

Pressed Density

The pressed density is measured as follows: 3 grams of powder is filled into a press mold with a diameter “d” of 1.300 cm. A uniaxial load of 2.8 t, corresponding to a pressure of 207 MPa, is applied for 30 seconds. After relaxing the load, the thickness “t” of the pressed powder is measured. The pellet density is then calculated as follows: 3/(π×(d/2)²×t) in g/cm³. The pressed density of the parent phases and examples have been measured and results are shown in Table 6.

TABLE 6 pressed density Avg. Pressed Particle density Sample Size (μm) (g/cm³) LCO-1 20 3.77 Ex1 20 3.76 Ex2 20 3.76 LCO-3 18 3.80 Ex3 16 3.90 LCO-4 6 3.44 Ex4 6 3.42 Electrochemical Performances

Electrochemical performances are tested in CR2032 coin type cells, with a Li foil as counter electrode in a lithium hexafluorite (LiPF₆) type electrolyte at 25° C. The active material loading is 10.5 (±0.5) mg/cm². Cells are charged to 4.3V and discharged to 3.0V to measure rate performance and capacity. The high voltage discharge capacity and capacity retentions during extended cycling are measured at 4.5V and 4.6V charge voltages. A specific capacity of 160 mAh/g is chosen for the determination of the discharge rates. For example, for the discharge at 2 C, a specific current of 320 mA/g is used. The following table 7 is a description of the test that is used for all of the coin cells in Examples 1 to 4.

TABLE 7 coin cell testing procedure for Examples 1 to 4 Cycle Charge Discharge number condition condition Test description 1 4.3 V cutoff 3.0 V cutoff Voltage profile at 0.1 C at 0.1 C 2 to 6 4.3 V cutoff 3.0 V cutoff Rate performance at 0.25 C at 0.2, 0.5, 1, 2 and 3 C 7 and 31 4.5 V (or 4.6 V*) 3.0 V cutoff Slow reference cycle cutoff at 0.25 C at 0.1 C before and after stability. Cycle 7 (31) gives the discharge capacity 0.1 C at 4.5 V (or 4.6 V*). 8 and 32 4.5 V (or 4.6 V*) 3.0 V cutoff Fast reference cycle cutoff at 0.25 C at 1 C before and after stability. Cycle 8 (32) gives the discharge capacity 1 C at 4.5 V (or 4.6 V*) 9 to 30 4.5 V (or 4.6 V*) 3.0 V cutoff Stability test at high cutoff at 0.25 C at 0.5 C voltage *as specified in the Examples

The following definitions are used for data analysis: Q: capacity, D: Discharge, C: Charge, followed by a number to indicate cycle number. For example, the slow high voltage discharge capacity DQ7 is measured during the 7^(th) cycle in the 4.5 (or 4.6)—3.0V range at 0.1C. The fast high voltage discharge capacity DQ8 is measured during the 8^(th) cycle in the 4.5 (or 4.6)—3.0V range at 1C.

Irreversible capacity Qirr(%) is ((CQ1−DQ1)/CQ1)×100.

Rate performance is defined by the ratio of DQ at respectively 0.2, 0.5, 1, 2 and 3 C versus DQ at 0.1C, converted to and expressed in %.

Capacity fade rate—expressed in %—at 0.1 C per 100 cycles is calculated as follows:

(1−(DQ31/DQ7))×100/23. Likewise, capacity fade rate at 1C per 100 cycles is: (1−(DQ32/DQ8))×100/23.

Energy fade rates at 0.1C and 1C are calculated in a similar way than capacity fade rate but instead of the discharge capacity DQ the discharge energy (defined as DQ×average discharge voltage) is used in the calculation.

The electrochemical performances of Examples 1-4 are shown in Table 8A. The electrochemical properties are unexpectedly improved compared to LCO-1, LCO-3 and LCO-4. At 4.3V, Ex1, Ex2, Ex3 and Ex4 show very small irreversible capacity and better rate performance compared to LCO-1, LCO-3 and LCO-4. 4.5V high voltage performances of Ex1, Ex2, Ex3 and Ex4 are enhanced and feature very high capacity and very good cycle life. 4.6V performances of Ex1, Ex2, Ex3 and Ex4 are exceptional with a capacity fading at 1C below 40% and are, to the extent of authors knowledge, unequalled in literature. These data show a perfect correlation between the relatively low defect concentration as evidenced by ⁷Li MAS NMR and the improved high voltage properties of stoichiometrically controlled LiCo_(1−x)M_(x)O₂.

The energy density at 0.1C for 4.5V and 4.6V, defined as the product of the pressed density, average voltage and discharge capacity, of Ex1, Ex2, Ex3 and Ex4 are enhanced when compared to LCO-1, LCO-2 and LCO-3, respectively. The association of high energy density and improved cycle life make Ex1, Ex2, Ex3 and Ex4 suitable for applications such as portable electronics. One can notice that the capacity fading of Ex4, though being significantly improved compared to LCO-4, is higher than Ex1, Ex2 and Ex3. This effect is a direct consequence of the lower particle size of Ex4 which causes a thinner surface layer—as the amount of added Mg and Ti is similar for all samples—offering less protection towards electrolyte decomposition.

TABLE 8A electrochemical properties at 4.5 V and 4.6 V. Capacity Capacity Energy 3 C rate fading at fading at density at Voltage Qirr performance DQ7 DQ8 0.1 C 1 C 0.1 C Experiment (*) (%) (%) (mAh/g) (mAh/g) (%) (%) (mWh/cm³) LCO-1 4.5-3.0 V 4.3 88.4 184.1 173.7 21.0 26.3 2807 4.6-3.0 V 202.1 181.2 130.9 164.2 3135 Ex1 4.5-3.0 V 1.9 93.2 190.4 187.7 0.0 2.1 2900 4.6-3.0 V 225.0 221.4 23.8 36.0 3485 Ex2 4.5-3.0 V 1.7 93.0 189.9 187.1 0.0 1.5 2893 4.6-3.0 V 224.4 220.8 12.2 24.7 3476 LCO-3 4.5-3.0 V 3.5 87.7 184.5 171.9 25.9 36.6 2834 4.6-3.0 V 194.2 174.7 115.2 162.2 3027 Ex3 4.5-3.0 V 2.5 91.6 189.7 184.1 0.4 2.0 3002 4.6-3.0 V 219.7 213.9 3.2 8.9 3532 LCO-4 4.5-3.0 V 2.0 94.7 189.6 184.5 11.68 15.62 2645 4.6-3.0 V 220.9 209.7 136.4 187.4 3134 Ex4 4.5-3.0 V 1.9 97.4 196.1 193.9 4.19 6.28 2724 4.6-3.0 V 229.2 225.5 95.2 138.7 3239 (*) referring to the Table under Measurement techniques where either 4.5 or 4.6 V is used as cutoff in charge conditions

Table 8B shows the discharge capacity, C-rate performances and average voltage at 4.4V of Ex4 and LCO-4. At 15C, the C-rate and average voltage of Ex4 are improved when compared to LCO-4 resulting in an increase of the specific energy Es, defined as the product of the average discharge voltage and discharge capacity, by about 4%. Ex4, by featuring improved cycle life and specific energy is highly suitable for high power applications where maintaining high specific energy at high C-rate is required.

TABLE 3B discharge capacity, C-rate properties, average voltage <V> at 15 C and specific energy Es for LCO-4 and Ex4 at 4.4 V. 1 C 5 C 10 C 15 C Es Sample DQ1 (%) (%) (%) (%) 20 C (%) <V> mWh/g LCO-4 172.9 97.3 94.6 93.2 91.9 90.1 3.76 597 Ex4 173.1 99.0 96.8 95.2 94.0 92.8 3.81 620

EXAMPLE 5

This example presents four lithium metal oxide compounds with different Li: Co stoichiometry and prepared by the following steps:

-   a) Li₂CO₃, Co₃O₄, MgO and TiO₂ powders are homogeneously mixed in     order to achieve a molar ratio of Li:Co:Mg:Ti of     1.10:0.9950:0.0025:0.0025. The mixture is then fired at 1010° C. for     12 h in an air flow of 4 m³/kg. The sintered product is milled and     classified resulting, in a powderous material with an average     particle size D50 is 16.1 μm. -   b) 13.04 mol % Co₃O₄ vs. Co content of product from step a) is added     into the product from step a). Then MgO and TiO₂ powders are added     to the mixture to achieve a molar content of 0.348%/0.283% vs. total     Co. Eventually, 8.56 mol % Li vs. total Co content is added to the     blend as Li₂CO₃. The mixture is fired at 980° C. in air for 10 h in     a box-type furnace. The black sintered powder is then ground and     sieved using a 270-mesh sieve. The mean particle size D50 is around     15 μm. -   c) Lastly 500 ppm of fumed alumina (Al₂O₃) nano-powder is added to     the product of step b). The final obtained powder is labeled Ex5-1.     The final Li:Co molar ratio is measured as 1.020 by ICP and the     electronic conductivity is 3.5×10⁻⁴ S/cm.

Example Ex5-2 is prepared in a similar way as Ex5-1, expect that 7.76 mol % Li vs. total Co content is added to the blend as Li₂CO₃. The final Li:Co molar ratio is measured as 1.011 by ICP and the electronic conductivity is 7.2×10⁻⁵ S/cm.

Example Ex5-3 is prepared in a similar way as Ex5-1, expect that 6.96 mol % Li vs. total Co content is added to the blend as Li₂CO₃. The final Li:Co molar ratio is measured as 1.005 by ICP and the electronic conductivity is 6.5×10⁻⁶ S/cm.

Example Ex5-4 is prepared in a similar way as Ex5-1, expect that 5.76 mol % Li vs. total Co content is added to the blend as Li₂CO₃. The final Li:Co molar ratio is measured as 0.993 by ICP and the electronic conductivity is 6.0×10⁻⁶ S/cm.

From sample Ex5-1 to Ex5-4, the Li to Co ratio gradually decreases from lithium in excess to lithium deficient. All samples are analyzed by XRD to check their phase composition. FIG. 17 shows the XRD patterns of Ex5-1 to Ex5-4 in a selected 2θ range of 20-55°, where the peak intensity is represented in the logarithmic scale with base 10, and the results are listed in Table 9. A careful phase analysis finds a cubic phase oxide having a crystal structure with a Fd-3mS space group—also referred to as spinet phase in this specification—in sample Ex5-4, as shown in FIG. 17, where the two indicated Bragg peaks belong to a phase with space group Fd-3mS. One peak is located at around 36.9° corresponding to Miller index (311), the other one at around 31.3° corresponding to Miller index (022). This spinet phase may contain Co, Mg, Ti and Al element with a formula M₃O₄ (M represents metal). The phase composition and the lattice constant of the secondary phase are listed in Table 10.

Sample Ex5-1 to Ex5-4 are analyzed in a coin cell test, where the voltage from step 7 to step 35 is set as 4.5V: the final LiCoO₂-based powders are electrochemically tested at a small scale in a half cell (=a coin cell). The half-cell is assembled by placing a separator (from SK Innovation) between the positive electrode and a piece of lithium metal as a negative electrode, and dropping the electrolyte of 1M LiPF₆ in EC/DMC (1:2) between separator and electrodes. All the cell tests of Examples 5 to 7 follow the procedure as shown in Table 9.

TABLE 9 coin cell testing procedure for Examples 5 to 7 Charge Discharge Cycle # C-rate E-Curr V C-rate E-Curr V 1 0.10 — 4.3 0.10 — 3.0 2 0.25 0.05 C 4.3 0.20 — 3.0 3 0.25 0.05 C 4.3 0.50 — 3.0 4 0.25 0.05 C 4.3 1.00 — 3.0 5 0.25 0.05 C 4.3 2.00 — 3.0 6 0.25 0.05 C 4.3 3.00 — 3.0 7 0.25  0.1 C 4.6 0.10 — 3.0 8 0.25  0.1 C 4.6 1.00 — 3.0 9-33 0.25 — 4.6 0.50 — 3.0 34  0.25  0.1 C 4.6 0.10 — 3.0 35  0.25  0.1 C 4.6 1.00 — 3.0

1 C-rate can be defined as 160 mAh/g. For example, 0.1C means that the cell will be charged or discharged in 10 hour. “E-Curr” and “V” stands for the end current and cut-off voltage, respectively. At the first cycle, the DQ0.1C (discharge capacity of the first cycle) and IRRQ (irreversible capacity) can be determined. The rate performance can be calculated from the subsequent five cycles. The performance of cycle stability is obtained from cycle #7 to #35.

Capacity fading at 0.1C and 1C are calculated as follows and are expressed in % per 100 cycles:

${0.1\mspace{14mu} C\mspace{14mu}{{QFad}.}} = {\left( {1 - \frac{{DQ}\; 34}{{DQ}\; 7}} \right) \times \frac{10000}{27}{in}\mspace{14mu}\%\text{/}100\mspace{14mu}{cycles}}$ ${1\mspace{14mu} C\mspace{14mu}{{QFad}.}} = {\left( {1 - \frac{{DQ}\; 35}{{DQ}\; 8}} \right) \times \frac{10000}{27}{in}\mspace{14mu}\%\text{/}100\mspace{14mu}{cycles}}$

Energy fading at 0.1C and 1C are calculated as follows and are expressed in % per 100 cycles. Vn is the average voltage at cycle n.

${0.1\mspace{14mu} C\mspace{14mu}{{EFad}.}} = {\left( {1 - \frac{{DQ}\; 34 \times \overset{\_}{V\; 34}}{{DQ}\; 7 \times \overset{\_}{V\; 7}}} \right) \times \frac{1000}{27}{in}\mspace{14mu}\%\text{/}100\mspace{14mu}{cycles}}$ ${1\mspace{14mu} C\mspace{14mu}{{EFad}.}} = {\left( {1 - \frac{{DQ}\; 35 \times \overset{\_}{V\; 35}}{{DQ8} \times \overset{\_}{V8}}} \right) \times \frac{1000}{27}{in}\mspace{14mu}\%\text{/}100\mspace{14mu}{cycles}}$

Retained discharge capacity at 0.1C and 1C refer to the DQ34 and DQ35 respectively.

In certain tests, the voltage of 4.6 V from step 7 to step 35 can be changed to 4.5 V, as mentioned in the Examples. The cyclability of the samples is summarized in Table 10. The results demonstrate that sample Ex5-4 possesses the best cyclability among those samples, so again the sample with the secondary spinet phase possesses the best cyclability.

TABLE 10 XRD phase analysis results and coin cell results of sample Ex5-1, Ex5-2, Ex5-3 and Ex5-4 Phase a_(s) (Å) of composition Q fading E fading Sample Fd-3mS (wt %) (%/100) (%/100) ID phase Fd-3mS R-3m 0.1 C 1 C 0.1 C 1 C. Ex5-1 — — 100 9 11.5 9 13.2 Ex5-2 — — 100 5.4 8.1 5.4 8.9 Ex5-3 — — 100 2.8 4.9 2.6 5.4 Ex5-4 8.097 1.10 98.90 1.3 1.5 1.1 1.3

EXAMPLE 6

This example presents three lithium metal oxide compounds with different Li: Co stoichiometry and prepared by the following steps:

-   a) Li₂CO₃, Co₃O₄, MgO and TiO₂ powders are homogeneously mixed in     order to achieve a molar ratio of Li:Co:Mg:Ti of     1.10:0.9950:0.0025:0.0025. The mixture is then fired at 980° C. for     12 h in an air flow of 4 m³/kg. The sintered product is milled and     classified, resulting in a powderous material with an average     particle size D50 is 16.7 μm. -   b) 13.04 mol % Co₃O₄ vs. Co content of product from step a) is added     into the product from step a). Then MgO, Al₂O₃ and TiO₂ powders are     added to the product of step a) to achieve a molar content of     0.348%/0.18%/0.283% vs. total Co. Eventually, 8.1 mol % Li vs. total     Co content is added to the blend as Li₂CO₃.

The mixture is again fired at 980° C. in air for 12 h in a box-type furnace. The black sintered powder is then ground and sieved using a 270-mesh sieve. The final obtained powder is labeled Ex6-1. The mean particle size is around 15 μm and the ICP Li:(Co+Al) molar ratio is 1.006 and the electronic conductivity is 5.4×10⁻⁵ S/cm.

Ex6-2 is prepared in a similar way as Ex6-1, expect that 6.9 mol % Li vs. total Co content is added to the blend as Li₂CO₃. The final Li:Co molar ratio is measured as 0.996 by ICP and the electronic conductivity is 1.4×10⁻⁶ S/cm.

Ex6-3 is prepared in a similar way as example Ex6-1, expect that 5.7 mol % Li vs. total Co content is added to the blend as Li₂CO₃. The final Li:Co molar ratio is measured as 0.986 by ICP and the electronic conductivity is 5.6×10⁻⁶ S/cm.

From sample Ex6-1 to Ex6-3, the Li to Co ratio gradually decreases from lithium in excess to lithium deficient. All samples are analyzed by XRD, and FIG. 18 shows their XRD patterns in a selected 2θ range of 30-55°. The phase compositions are analyzed as in the previous examples. The results are listed in Table 11. This phase analysis finds a spinet phase in sample Ex6-3, as shown in FIG. 18: two indicated Bragg peaks belong to a cubic phase oxide with space group Fd-3mS. The peak at around 36.8° corresponds to Miller index (311), the other at around 31.2° corresponds to Miller index (022). This spinet phase may contain Co, Mg, Ti and Al and have a general formula M₃O₄ (M represents metal). The phase composition and lattice constant of the secondary phase are listed in Table 11.

The cyclability of the samples is tested in the coin cell test described in Ex. 5 (with a voltage of 4.6 V from step 7 to step 35), the results are given in Table 11. It is observed that sample Ex6-3 possesses the best cyclability and is the only sample containing the spinet phase. Thus, it is believed that a secondary spinet phase with the XRD patterns described above could reduce the capacity fading during cycling of LiCoO₂-based powders are applied as cathode material.

TABLE 11 XRD phase analysis results and coin cell results of sample Ex6-1, Ex6-2 and Ex6-3 a_(s) (Å) of Phase Fd- composition Q fading E fading Sample 3mS (wt %) (%/100) (%/100) ID phase Fd-3mS R-3m 0.1 C 1 C 0.1 C 1 C. Ex6-1 — — 100 101.52 160.94 113.00 177.05 Ex6-2 — — 100 28.04 51.95 31.62 61.16 Ex6-3 8.101 1.47 98.53 18.49 37.26 20.76 43.90

EXAMPLE 7

This example will demonstrate that the synergetic effect between the cubic phase and the layered Co-based phase exists only when the process according to the invention is applied.

Preparation of the LCO-7 core: 0.25 mol % Mg-doped and 0.083 mol % Ti-doped LiCoO₂ is prepared at pilot scale by mixing MgO, TiO₂, Li₂CO₃ and Co₃O₄. The blend is put in a mullite sagger and fired at 995° C. for 10 h in an air-flow atmosphere. The fired product is then crushed and classified and results in a powder having a D50=23.6 μm and a Li:Co molar ratio or 1.069.

Preparation of Ex7a: 86.96 mol % of LCO-7 is mixed with 13.04 mol % of Co₃O₄ powder with D50=3 μm, with Al₂O₃, MgO and TiO₂ powders with target molar ratios of Al/Co=0.180, Mg/Co=0.348 and Ti/Co=0.283, and with 6.1 mol % Li (as Li₂CO₃) vs. the molar content of Co+Al is introduced to the mixture. The mixture is fired at 980° C. for 8.5 h in air. The fired product is then crushed and classified and results in a powder having a specific surface area of 0.20 m²/g, a packing density of 4.01 g/cc under 207 MPa and an ICP-determined Li:(Co+Al) molar ratio of 0.963; and the electronic conductivity is 2.7×10⁻⁵ S/cm. The XRD pattern, showing the diffracted intensity in log 10-scale as function of the 2θ angle of Ex7a is shown on FIG. 19. The main peaks correspond to a Co-based layered phase with space group R-3m. In addition, additional peaks at 31.2°, 36.8° and 55.6° 2θ correspond to a cubic spinet phase oxide having a crystal structure with a Fd-3mS space group (space group number 227), with a_(s)=8.099 Å. The wt % content of the second phase is estimated to 4.92 wt % based on Rietveld refinement. Doping by either one or more Mg, Ti and Al of the Co-based secondary phase is also not excluded.

Preparation of example 7b: Ex7b is prepared in a similar way as Ex7a, expect that 7.1 mol % Li (as Li₂CO₃) vs. the molar content of Co+Al is introduced to the mixture. Ex7b results in a powder having a specific surface area of 0.20 m²/g, a packing density of 4.01 g/cc under 207 MPa and an ICP-determined Li:(Co+Al) molar ratio of 0.990 and the electronic conductivity is 2.4×10⁻⁶ S/cm. The XRD pattern, showing the diffracted intensity in log 10-scale as function of the 2θ angle is shown in FIG. 19. The main peaks correspond to a Co-based layered phase with space group R-3m. In addition, additional weak but visible peaks at 31.2° and 36.8° 2θ correspond to a cubic phase oxide having a crystal structure with a Fd-3mS space group, with a_(s)=8.111 Å. The wt % content of the secondary phase is estimated to 0.75 wt % based on Rietveld refinement. Doping by either one or more of Mg, Ti and Al of the Co-based secondary phase is also not excluded.

Preparation of example 7c: Ex7c is prepared in a similar way as Ex7a, expect that 8.1 mol % Li (as Li₂CO₃) vs. the molar content of Co+Al is introduced to the mixture. Ex7c results in a powder having a specific surface area of 0.19 m²/g, a packing density of 4.06 g/cc under 207 MPa and an ICP-determined Li:(Co+Al) molar ratio of 0.999 and the electronic conductivity is 4.9×10⁻⁵ S/cm. The main peaks correspond to a Co-based layered phase with space group R-3m. No secondary phase is observed.

Preparation of example 7d: Ex7d is prepared by mixing 98 wt % of Ex7c powder with 2 wt % of Co₃O₄ powder. The mixture results in an ICP-determined Li:(Co+Al) molar ratio of 0.980. The XRD pattern, showing the diffracted intensity in log 10-scale as function of the 2θ angle of Ex7d is shown on FIG. 19. The main peaks correspond to a Co-based layered phase with space group R-3m. In addition, additional peaks at 31.2°, 36.8° and 55.6° 2θ correspond to a cubic spinet phase oxide having a crystal structure with a Fd-3mS space group, with a_(s)=8.089 Å. The wt % content of the cubic phase is estimated to 2.86 wt % based on Rietveld refinement, in good agreement with the nominal composition of 2 wt %.

Next, electrodes prepared with powders of Ex7a-d are fitted in coin cells. The electrochemical results at 4.6V vs. Li-metal are shown in the Table 12.

TABLE 12 4.6 V coin cell results of samples 7a-d Capacity at 1 C 4.6 V capacity fading 4.6 V energy fading Exam- (DQ8) (%/100) (%/100) ple mAh/g 0.1 C 1 C 0.1 C 1 C 7a 209.0 8.6 18.9 9.3 21.7 7b 211.9 10.7 19.7 11.8 23.7 7c 210.3 61.3 103.8 70.2 119.2 7d 206.7 62.0 101.6 70.9 118.8

It is observed that:

-   -   (i) The cycle stability is significantly increased when the         Co-based secondary spinel phase content increases from zero in         Ex7c to nearly 5 wt % in Ex7a.     -   (ii) Example 7d has exactly 98% of the DQ8 value of Ex7c as         expected from the mixing ratio of 98 wt % of electrochemically         active Ex7c powder and 2 wt % of electrochemically inactive         Co₃O₄ spinel powder. Although containing a large amount of         Co-based spinel, no improvement of cycle life is observed for         Ex7d vs. Ex7c.     -   (iii) Examples 7a and 7b show a less-than-expected DQ8 capacity         loss: assuming that full capacity is achieved for Ex7c and that         the electrochemically active Co-based layered materials weight         content are respectively 95.08% and 99.25%, one would expect         that Ex7a and 7b have respective DQ8 capacities of 208.7 and         200.0 mAh/g. Experimental results show that the capacity is         surprisingly and significantly higher for Ex7a and 7b.     -   (iv) No synergetic effect exists between the Co-based layered         phase and the Co-based cubic secondary phase when the Co-based         cubic secondary phase is introduced by simple blending and         without extra heat-treatment process step.     -   (v) The dramatic improvement of cycle stability at 4.6V for         examples 7a and 7b compared to examples 7c and 7d can be         ascribed to two main contributions:         -   1_(—) partial or full Mg and Ti extrusion and accumulation             on the surface of the particles following the mechanism             discussed in Example 1 when decreasing the lithium to metal             ratio. Accumulation of Ti and Mg on the surface of the             particles are supposedly improving the cycle stability by             providing protection against electrolyte oxidation and             subsequent cathode materials degradation such as oxygen loss             or other structural collapses.         -   2_(—) the Co-based cubic secondary phase is partially or             fully spatially distributed on the surface of the surface of             the Co-based layered particles only in samples 7a and 7b.             The Co-based cubic secondary phase may or may not             homogeneously cover the particle surface. Ideally, the             coverage is not homogeneous in order to prevent an increase             in cathode polarization. The presence of Co-based cubic             secondary phase on the surface of the Co-based layered             particles is also supposed to provide protection against             cathode materials degradation such as oxygen loss or other             structural collapses resulting in an improved cycle             stability at 4.6V. 

The invention claimed is:
 1. A lithium metal oxide powder for a cathode material in a rechargeable battery, comprising a core material and a surface layer, the core material having a layered crystal structure consisting of the elements Li, a metal M and oxygen, wherein the metal M has the formula M=Co_(1−a)M′_(a), with 0≤a≤0.05, wherein M′ is selected from one or more metals of the group consisting of Al, Ga and B; and the surface layer comprising a mixture of the elements of the core material Li, M and oxygen, inorganic N-based oxides and a cubic phase oxide having a crystal structure with a Fd-3mS space group, wherein N is selected from one or more metals of the group consisting of Mg, Ti, Fe, Cu, Ca, Ba, Y, Sn, Sb, Na, Zn, Zr and Si.
 2. The lithium metal oxide powder of claim 1, wherein the Li content is set at a value of Li:M of 0.995 or below.
 3. The lithium metal oxide powder of claim 2, wherein the Li content is set at a value of Li:M between 0.963 and 0.993.
 4. The lithium metal oxide powder of claim 1, wherein the cubic phase oxide having a crystal structure with a Fd-3mS space group has a lattice constant “as ” with 8.055≤a_(s)≤8.150, a_(s) being expressed in angstom (Å).
 5. The lithium metal oxide powder of claim 1, wherein the amount of cubic phase oxide in the lithium metal oxide powder is between 0.1 and 5 wt %.
 6. The lithium metal oxide powder of claim 1, wherein the lithium metal oxide powder has an electrical conductivity less than 10⁻⁵ S/cm, the electrical conductivity being measured under an applied pressure of 63.7 MPa.
 7. The lithium metal oxide powder of claim 1, having a ⁷Li MAS NMR spectrum that features a unique resonance around −0.5 ppm.
 8. The lithium metal oxide powder of claim 1, wherein 0<a≤0.03.
 9. The lithium metal oxide powder of claim 1, wherein the thickness of the surface layer is less than 100 nm.
 10. The lithium metal oxide powder claim 1, wherein N consists of Mg and Ti, the Mg content being between 0.1 and 1 mol % and the Ti content between 0.1 and 0.5 mol %.
 11. The lithium metal oxide powder claim 1, wherein the N-based oxides comprise oxides having incorporated Li atoms.
 12. The lithium metal based powder of claim 1, wherein the core material is substantially free from the paramagnetic metals Co²⁺, intermediate spin Co³⁺ and Co⁴⁺.
 13. The lithium metal oxide powder of claim 1, wherein the powder has a bimodal particle shape distribution where the small particle size fraction has a D50≤5 μm and is between 3 to 20 Vol %, and where the large particle size fraction has a D50≥12 μm.
 14. The lithium metal oxide powder of claim 1, wherein a concentration of N in the surface layer decreases with increasing distance from the surface of the lithium metal oxide powder.
 15. A method for manufacturing a lithium metal oxide powder of claim 1, the method comprising: providing a first mixture of a first Co- or Co and M′-comprising precursor powder and a first Li-comprising precursor powder, the first mixture having a Li to M molar ratio >1.01, sintering the first mixture in an oxygen comprising atmosphere at a temperature T₁ of at least 600° C., thereby obtaining a Li-enriched lithium metal oxide compound, providing a second Co- or Co and M′-comprising precursor powder, and mixing the Li-enriched lithium metal oxide compound and the second Co- or Co and M′-comprising precursor powder, thereby obtaining a second mixture wherein the molar ratio of Li to M is ≤0.995, and sintering the second mixture in an oxygen comprising atmosphere at a temperature T₂ of at least 600° C.; wherein one or more of the first Co- or Co and M′-comprising precursor powder, the first Li-comprising precursor powder, or the second Co- or Co and M′-comprising precursor powders further comprises at least one element selected from the group consisting of Mg, Ti, Fe, Cu, Ca, Ba, Y, Sn, Sb, Na, Zn, Zr and Si.
 16. The method according to claim 15, wherein the first mixture has a Li to M molar ratio between 1.02 and 1.12.
 17. The method according to claim 15, wherein the second mixture has a ratio of Li to metal Li:M between 0.963 and 0.993.
 18. An electrochemical cell comprising a cathode comprising the lithium metal oxide powder according to claim
 1. 